PV Arc Fault Trip Recurring After Clear Decision Tree

Why this matters

A PV inverter that trips on a DC arc-fault, clears manually, runs for a day or two, then trips again is signaling a real intermittent arc somewhere in the array. The arc-fault circuit interrupter (AFCI) function required by NEC 690.11 is doing its job. Ignoring or repeatedly bypassing the trip puts the array in an unsafe state - PV arcs are sustained, high-energy, and ignite roof underlayment in minutes. The decision tree below isolates the source (loose MC4 connector, damaged conductor insulation, junction-box terminal, or false trigger from RFI) so the next service visit identifies and corrects the arc - not just resets the inverter.

Symptom presentation

Inverter reports arc fault, often with a code like AFCI Fault, Arc Detected, Earth Fault Plus Arc. Manual reset clears the trip and the system runs normally for hours to days. Trip then recurs. Customer or O&M provider has reset the system multiple times. Sometimes a weather correlation appears (trips after thunderstorms or on humid mornings); sometimes the trip is purely time-of-day driven (mid-irradiance trips when current is rising).

Quick checks before reset

1. Do not blindly reset. Per NEC 690.11(B), the inverter must require manual reset after an arc-fault interruption. Repeated reset without inspection violates the intent of the AFCI rule and risks an under-investigation fire.
2. Read inverter logs and any string-level DC voltage and current data. A trip that happens at a specific irradiance level (sustained high DC current) localizes to a current-loaded fault path.
3. Look at the array exterior with an IR camera at the next irradiance peak. Hot spots at connector pairs, terminal lugs, or junction boxes are the most common arc precursors.
4. Confirm AFCI sensitivity matches OEM spec. SolarEdge, SMA, Enphase, and Fronius all expose the AFCI threshold; an aftermarket setting that was lowered for an unrelated reason can also cause nuisance trips.
5. Look at the conductor runs for visible damage, rodent activity, or roof-flashing penetration.

Isolation tree

Step 1: Walk the array and inspect every MC4 connector pair for separation, scorch marks, or wetness. Pull each connector and look at the pin for blackening. A burned pin confirms arcing at that interface. Replace the connector pair with the OEM-matched part.

Step 2: Run an IR camera scan during the next irradiance window above 500 W/m^2. Hot spots at conductor pairs, fuse holders in the combiner, or module junction boxes confirm series-resistance fault paths. A connector running 20 C above ambient is a candidate arc source.

Step 3: Inspect the home-run conductors for damage. PV wire (USE-2 or PV wire under UL 4703) that has been stapled against a sharp edge or pinched at a flashing has insulation damage that creates wet-condition arc paths. Look at flashing penetrations on tile and shake-shingle roofs especially.

Step 4: Inspect the disconnect and the combiner box. Loose lug torque drops at thermal cycles; an annual re-torque to spec catches this. Aluminum-to-copper lug junctions without proper antioxidant compound oxidize and arc.

Step 5: Test for RFI false triggers. SolarEdge and SMA inverters can false-trip on nearby high-power RF sources (cell tower interaction, ham radio, defective LED driver in the home). If no physical arc source can be identified after a thorough walk and IR scan, contact OEM for a firmware update or RF immunity assessment.

Step 6: For rapid-shutdown-enabled systems (NEC 690.12), inspect the Module-Level Power Electronics (MLPE) units. A failing optimizer or microinverter draws inconsistent current under load and can present as an AFCI fault.

Confirming diagnosis

Burned connector confirms by visible damage and arc-track on the pin. Insulation damage confirms by inspection at the suspected wear point. Loose lug confirms by re-torque measurement; the spec is on the lug tag. MLPE fault confirms by module-level data at the time of trip. RFI false trigger confirms by elimination of physical causes plus OEM-confirmed firmware-update path.

Remediation

For burned MC4: replace as a pair. Use only matched-brand connectors per UL 6703 - mixing Stäubli, Amphenol, and clones is a recurrent root cause.

For insulation damage: cut out the damaged section and splice with PV-listed junction or replace the conductor. Use the right strain relief at flashing penetrations.

For loose lugs: torque to spec, use antioxidant compound on aluminum-to-copper joints per the lug manufacturer.

For MLPE fault: replace the unit through the OEM warranty path. Enphase IQ8, SolarEdge optimizers, Tigo TS4 all have RMA processes.

For RFI false trigger: update inverter firmware per OEM. If trip persists, escalate to OEM engineering.

After repair, run two weeks without trip before closing the case. Document with photos at every connector and lug touched.

References

1. NEC 2023 Article 690.11 Arc-Fault Circuit Protection for PV Systems.
2. NEC 2023 Article 690.12 Rapid Shutdown of PV Systems on Buildings.
3. UL 1699B Outline of Investigation for Photovoltaic DC Arc-Fault Circuit Protection.
4. UL 6703 Standard for Connectors for Use in Photovoltaic Systems.
5. SolarEdge HD-Wave Inverter Installation Guide and SMA Sunny Boy US Service Manual on AFCI behavior and reset procedures.